Ciliates keep two genomes: one for heredity, a shredded copy for daily life
Each ciliate carries a small nucleus that it passes to offspring but never reads, and a huge working nucleus built by chopping, copying and splicing that inherited DNA. In some species more than 95 percent of the germline genome is thrown away in the process, and genes must be unscrambled to work.
Ciliates are single-celled protists covered in cilia, hair-like structures built like flagella but shorter, more numerous and beating in a different rhythm. They use them to swim, crawl, cling, feed and sense. They turn up almost anywhere wet, from oceans to soil and even oxygen-starved mud. Around 4,500 free-living species have been described, with perhaps 27,000 to 40,000 in existence, ranging from 10 micrometres to 4 millimetres long.
The two nuclei split the work. The tiny diploid micronucleus is the germline; the large macronucleus runs the cell. To build the macronucleus, the cell fragments the micronuclear chromosomes, cuts out intervening stretches and copies the pieces many times. In Tetrahymena, 10 chromosomes become over 20,000, many holding a single gene, and about 15 percent of the DNA is discarded along the way. In spirotrichs such as Oxytricha, gene segments sit out of order or backwards, and long RNAs from the parent's macronucleus guide the reassembly.
They multiply by splitting in two, but sex is separate from reproduction. In conjugation, two compatible partners form a bridge, their micronuclei undergo meiosis, and they swap haploid nuclei. Each then rebuilds a fresh macronucleus. Without this reset, cells age: a clone of Paramecium tetraurelia dies out after about 200 divisions. Transplant experiments by Aufderheide in 1986 pinned ageing on the macronucleus rather than the cytoplasm, and later work linked it to accumulating DNA damage.
Most ciliates sweep bacteria, algae and debris into a mouth groove with specialised cilia, digesting them in food vacuoles and expelling leftovers through an anal pore; star-shaped contractile vacuoles in Paramecium pump out excess water. Others hunt fellow ciliates, absorb nutrients directly or borrow photosynthesis from captured microbes, and Halteria has been seen eating viruses. Only one species, Balantidium coli, is known to cause human disease. Claimed 580-million-year-old fossils turned out to be something else, but a Vorticella preserved in a Triassic leech cocoon dates back about 200 million years.
Source: Ciliate
